This disclosure relates to gas conditioning systems for non-contacting gas seals. More particularly, it relates to a system for monitoring and control of seal buffer gas.
Non-contacting seals for gas compressors and other rotating equipment such as gas and steam turbines, turbo expanders, centrifugal pumps and the like, operate on a thin film of conditioned process gas; pre-treated to render it suitable for delivery to, and passage through, the seal mechanism. Commonly, the source of this seal gas, sometimes called buffer gas, is the machine discharge.
The principle of dry gas seal technology is that the sealing faces are non-contacting and a clean and dry gas is allowed to pass through the seal interface. It flows from the high pressure side of the seal to the low pressure seal and is routed to a flare line through the primary vent outlet module which comprises monitoring instruments and a safety trip to shut down the compressor in the event of high seal leakage. Typically abnormal seal gas leakage has represented the sole measure of seal performance.
Seal gas, that is the gas upon which the non-contacting seal operates, is process gas usually from the discharge line of the compressor unit, piped to the control system supply line. The control system then regulates and filters the buffer gas flow before it is injected to the primary seal chamber. The pressure and leakage flow rate are monitored and recorded to ensure that the seals function properly.
A known cause of seal failure is a lack of clean and dry seal or buffer gas being supplied to the compressor. Critical to gas seal longevity, seal gas must be free of liquid vapor or condensate. Liquid contamination has been found to be a leading cause of the failures. Particular applications prone to liquid contamination were mostly found on offshore platforms, Hydrogen Recycle, Gas Gathering, Ammonia, HP pipelines and similar seal applications. Initial gas composition information is often unreliable, and changes with time, resulting in failures due to liquid condensation.
Attention to reliability and damage prevention is particularly critical because of the requirements of high-pressure compressors used in exploration, such as gas reinjection and the complexity of gas compositions involved. Unexpected seal failures cause operational loss and delay in start-up.
Also, initial system selection often sabotages optimal reliability. Compressor manufacturers often do not review the make-up of the process fluid, including gas composition, operating pressure and temperature, liquid and contaminate level in the process gas, and the auxiliary buffer gas requirement. Additionally, current systems do not offer an advanced warning or initiate corrective action to prevent exposure to free liquid or condensate, which it is considered to be a major root cause of failures. The current method of seal health evaluation based on leakage volume is insufficient. And, failures are costly because of the delay in plant start-up and loss of production.
As compressor operating requirements push past current limits, there is a clear need for innovative and intelligent approaches to support the emerging compressor markets.
One way to improve the reliability of these new designs is to integrate such seals with imaginative control system technology. Achieving optimum reliability is assured by providing appropriate control system technology to ensure that clean and dry buffer gas is always available to the non-contacting faces of the seal.
Prior efforts to monitor seal gas have focused on recognition of conditions within the seal chamber containing the non-contacting seal devices. One example is disclosed in U.S. application for patent Ser. No. 12/469,045 filed May 20, 2009 (Publication US2009/0290971) the entire specification and drawings of which are hereby incorporated by reference herein as if fully set forth.